1. Field of the Invention
The present invention relates to an image processing method and apparatus for compressing image data in an adaptive manner to an input image.
2. Related Background Art
As one of several techniques for compressing multi-value images, there has been proposed an ADCT (Adaptive Discrete Cosine Transform) compression method mainly applied to natural images. With this compression method, three primary (RGB) signals are converted into three components of Y, U and V. Of these components, the Y signal representing luminance is compressed at the same resolution, while the U, V signals representing chrominance components are compressed at a lowered resolution after sub sampling. In a step 1 of the compression, each component is subjected to DCT in units of 8.times.8 pixel blocks for conversion into a frequency space of 8.times.8 to thereby obtain DCT coefficients. In a step 2, quantization tables are respectively prepared for the luminance component (Y) and the chrominance components (U, V) so that the DCT coefficients are linearly quantized (divided) for each component using quantization values of 8.times.8 which are produced by multiplying quantization factors by respective elements of the 8.times.8 quantization table, thereby obtaining quantization coefficients. In a step 3, quantization coefficients are subjected to Huffman encoding.
However, when an image comprising a natural image area, a color character image area and a CG image area in a mixed pattern, for example, is compressed by using the prior art compression method as stated above, there is a severe shortcoming in that the quality of the color character image and the CG image deteriorates.
Also, it has not been conceived to separately perform quantization of the luminance data and the chrominance data in respective suitable manners.